Eyepiece selection based on exit pupil

[Rob_UK_SE]

17 minutes ago, amaury said:

your three EP set plus a barlow is what I am planning more or less. I'm about to buy the APM Ultraflat field 30mm 70 AFOV . Price scales up rather quick with those premium 80 AFOV eyepieces. I believe that eyepiece is a good balance between optical quality, field of view, exit pupil, eye relief and value.

That’s a very good eyepiece and I’m sure that you would be very happy with it.  It competes very well with offerings that are priced quite a lot higher than the APM.

[amaury]

2 hours ago, John said:

My skies are around bortle 5.

With my 12 inch F/5.3 dobsonian I have a couple of low power / wide field options:

- 31mm / 82 degrees which gives 51x, a true field of 1.6 degrees and an exit pupil of 5.84mm

- 21mm / 100 degrees which gives 76x, a true field of 1.32 degrees and an exit pupil of 3.96mm

Most of the time I find the 21mm is the most effective eyepiece for picking out faint deep sky objects.

I've had a number of galaxy hunting sessions where the 21mm is the only eyepiece I've used in fact.

I'll keep that in mind. A very wide EP would be nice to have at 4mm exit pupil. I believe the 21mm/100 degrees you mention is the TV Ethos. Way beyond my budget I am afraid.

[amaury]

2 hours ago, Rob_UK_SE said:

Hello amaury,

I most frequently observe from our garden which benefits from Bortle 4 skies. For low power views of large objects (m81 and m82 together, the Pleiades, m31, the Veil, Double Cluster etc.)  I can only echo John’s comments that I too prefer using a 21mm eyepiece. This results in an exit pupil of 4.2mm. I also have a 30mm, but find the contrast is less effective from home when compared to the 21mm. By contrast (excluding the terrible pun), when I travel to darker skies I find the larger exit pupil useful, but it is subtle to my eyes at least. The darker skies have a much more dramatic effect (comparing the 21mm at home vs away). I have recently ordered a new scope so it will be interesting to see the impact of the new focal ratio on these eyepieces as I am going from f5 to f4.6.

I think if I was observing in a Bortle 6 area I would probably be aiming for approximately a 3mm exit pupil as my low power option (16mm or 17mm).

Just to complicate matters further, it’s also worth considering the impact of an eyepiece’s light transmission, scatter and the quality of the coatings as they all play a part in the quality of views and contrast achieved too.

Rob

Thanks Rob. (that was a funny pun tho'). I certainly understand there will be better contrast with the 21mm EP in bortle 4/5 skies. The thing is the balancing act between price/TFOV/Exit Pupil etc. Those 21mm with 100 AFOV EPs have a really steep price, as much as I would love to have one of those, I don't want to spend 80% of my budget on a single piece. I'll keep them in mind for a future investment for sure. 

[Rob_UK_SE]

If it helps, I purchased a Pentax XW 30mm about a year ago, but have fairly recently sold it and purchased the cheaper APM 30mm ultra flat (which I consider even better). It was highly recommended at the time, but I secured a good deal on the XW... I wish I’d followed the advice on here and gone for the APM from the start!   

In comparison with the formidable 31mm Tele Vue Nagler, the 31mm has a considerable pincushion distortion, whereas the 30mm APM UFF is much more comfortable when scanning through the Milky Way etc.

The Baader Morpheus would work well alongside your future APM too.

Edited April 11, 2021 by Rob_UK_SE

[amaury]

11 minutes ago, Rob_UK_SE said:

If it helps, I purchased a Pentax XW 30mm about a year ago, but have fairly recently sold it and purchased the cheaper APM 30mm ultra flat (which I consider even better). It was highly recommended at the time, but I secured a good deal on the XW... I wish I’d followed the advice on here and gone for the APM from the start!   

In comparison with the formidable 31mm Tele Vue Nagler, the 31mm has a considerable pincushion distortion, whereas the 30mm APM UFF is much more comfortable when scanning through the Milky Way etc.

The Baader Morpheus would work well alongside your future APM too.

I have read many good things about the APM too, that it punches above its weight, but never thought it would be better than a Pentax XW. Those are premium EPs as far as I know. The Baader Morpheus will be the 12.5mm (aiming for the "sweet spot" of 2mm exit pupil) Thank you all for your insight. It is appreciated indeed.

[mikeDnight]

5 hours ago, wookie1965 said:

Mike did you make your own or buy the hood I could do with one from home. 

There is a company who makes observing hoods, but I don't have one of those. I use a blackout blanket, which is actually a black pet blanket. I've thought about turning it into an actual hood, but haven't yet got round to it. I've only had it ten years! It is very effective in helping with dark adaption, as after half an hour under the blanket observing, my surrounding, normally dark garden, becomes blindingly bright . 

[johninderby]

I have thought that one of these would make a very good observing hood? 🤔

https://www.ebay.co.uk/itm/US-ARMY-M1951-PARKA-HOOD-m51/313411641141?hash=item48f8ca1f35:g:QTIAAOSwHDNgIU9u

[Stu]

2 hours ago, amaury said:

I'll keep that in mind. A very wide EP would be nice to have at 4mm exit pupil. I believe the 21mm/100 degrees you mention is the TV Ethos. Way beyond my budget I am afraid.

The APM 20mm XWA is 100 degree and has an excellent reputation as being pretty close to a 21mm Ethos for a lot less money. Worth a look.

APM HDC - XWA 20 mm 100° Eyepiece
https://www.apm-telescopes.de/en/optical-accessories/eyepieces/more-74-ultra-wide-angle/apm-lunt-eyepieces/apm-hdc-xwa-20-mm-100-eyepiece.html

 

[faulksy]

6 hours ago, wookie1965 said:

Mike did you make your own or buy the hood I could do with one from home. 

you beat me to it paul 😁😁

[Louis D]

Alright, I'll come to the defense of 40mm SWA eyepieces at f/6 under Bortle 6 skies.  I enjoy maxing out the field of view while viewing large objects like the Pleiades or Collinder 70.  They're bright enough that the brighter sky background isn't distracting at all.  They're also enjoyable for sweeping rich star fields.

Yes, they're marginally useful for detecting smaller DSOs due to the low contrast with the brighter sky background, so higher power and wider field of view eyepieces are more useful.

[amaury]

10 hours ago, Stu said:

The APM 20mm XWA is 100 degree and has an excellent reputation as being pretty close to a 21mm Ethos for a lot less money. Worth a look.

Thanks Stu, looks a like a bargain compared to a 21mm ethos or a ES100.  

I reckon I have to consider import fees when buying from Europe now. Is it any different than importing something from the US? 

I can get a similar TFOV with an ES82 24mm, a bit better eye relief and a tad cheaper than the APM 20mm

[amaury]

6 hours ago, Louis D said:

so higher power and wider field of view eyepieces are more useful

I'll pay the premium for usefulness. Thanks Louis.

[Stu]

21 minutes ago, amaury said:

I reckon I have to consider import fees when buying from Europe now. Is it any different than importing something from the US? 

It’s well worth checking it out carefully. I did email Astroshop.eu the other day, and they reassured me by way of this reply:

We offer a special service for our UK customers whereby we offer greatly reduced shipping costs and pay any incurrable customs fees. This means, you don't have to deal with customs paperwork and further payment at your end!
In this case the VAT will not be deducted.

Still not totally sure it would be without additional cost or hassle but I guess we won’t know until we’ve tried! Check with each individual retailer though.

Astrograph also list the same eyepiece in the U.K. which is likely a safer bet but they are not showing any stock at the moment.

https://astrograph.net/epages/www_astrograph_net.mobile/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/AGLE20HDC&Locale=en_GB

They are excellent eyepieces. I had one and sold it, but considering getting another soon when funds permit.

[Deadlake]

33 minutes ago, Stu said:

It’s well worth checking it out carefully. I did email Astroshop.eu the other day, and they reassured me by way of this reply:

We offer a special service for our UK customers whereby we offer greatly reduced shipping costs and pay any incurrable customs fees. This means, you don't have to deal with customs paperwork and further payment at your end!
In this case the VAT will not be deducted.

Still not totally sure it would be without additional cost or hassle but I guess we won’t know until we’ve tried! Check with each individual retailer though.

Astrograph also list the same eyepiece in the U.K. which is likely a safer bet but they are not showing any stock at the moment.

https://astrograph.net/epages/www_astrograph_net.mobile/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/AGLE20HDC&Locale=en_GB

They are excellent eyepieces. I had one and sold it, but considering getting another soon when funds permit.

Astrograph have a shipment in at the end of April, pending APM's large shipment to come in from KUO. I'm waiting on some APM EP's which I got before the price rise. As far as performance goes, when I purchased I discussed with another SGL user who has both the E21 and the APM 20 XWA, he told me skip the E21, the APM is sharper in the middle, weighs half as much and only losses out to the E21 with a small amount of astigmatism at the edge which you would notice unless under SQM > 21.8 skies. So I did, also got the APM 30 UFF as will show more contrast on darker skies.  Hope that helps.

Edited April 12, 2021 by Deadlake

[John]

I agree that the APM XWA 20mm would be an excellent alternative to the 21mm Ethos and at considerably lower cost.

Originally I had the Explore Scientific 20mm / 100 then had the opportunity to obtain a 21mm Ethos at a very reasonable cost so could not resist. The differences between the ES 20 / 100 and the Ethos 21 are small - mostly that I can take in the full field of the Ethos without rolling the eye cup down. I imagine that the differences between the Ethos 21 and the APM XWA 20mm are even smaller. Hey, ho, I have the 21mm Ethos now so I'll stick with it.

I deliberately try not to name the specific equipment brands that I use now because "naming the names" tends to lead to rather dull "this brand vs that brand" digressions. Probably been involved in too many of those over the years

With hindsight I should have posted that the eyepiece concerned are  20mm-ish / 100 and 30mm-ish / 82 eyepieces.

There are a number of ~20mm / 100 degree eyepiece options around. I think the point I was making was that eyepieces of this specification offer something very useful in terms of exit pupil / true field / magnification 

 

 

Edited April 12, 2021 by John

[amaury]

1 hour ago, Stu said:

It’s well worth checking it out carefully

I'll do my homework with this. 

1 hour ago, Deadlake said:

Astrograph have a shipment in at the end of April, pending APM's large shipment to come in from KUO. I'm waiting on some APM EP's which I got before the price rise. As far as performance goes, when I purchased I discussed with another SGL user who has both the E21 and the APM 20 XWA, he told me skip the E21, the APM is sharper in the middle, weighs half as much and only losses out to the E21 with a small amount of astigmatism at the edge which you would notice unless under SQM > 21.8 skies. So I did, also got the APM 30 UFF as will show more contrast on darker skies.  Hope that helps.

It helps yes. The thing is that with my scope (8" 1200mm f/6 Dob), the APM 20 gives me 1.67deg TFOV, that makes the APM 30 UFF redundant as it almost overlaps in terms of TFOV (1.75 TFOV for the UFF).

The one thing I would be giving up with the 20/100 is eye relief (15mm). I only have very small corrections (-0.5 Sph and -0.5 Cyl) so I think I can do visual observing without glasses. Maybe I am wrong here and I am being too optimistic. 

42 minutes ago, John said:

eyepieces of this specification offer something very useful in terms of exit pupil / true field / magnification

I totally understand that usefulness now. I just never considered  ~20mm / 100 degree eyepieces because of the massive price premium of those specs. I was aiming more towards 24mm/82 deg or 30mm/70 because they have an equivalent TFOV in my scope, better eye relief, and better prices, at the expense of more exit pupil (which wouldn't be an issue in truly dark skies, but it doesn't get better than bortle 4 for me).   

[Deadlake]

26 minutes ago, amaury said:

I'll do my homework with this. 

It helps yes. The thing is that with my scope (8" 1200mm f/6 Dob), the APM 20 gives me 1.67deg TFOV, that makes the APM 30 UFF redundant as it almost overlaps in terms of TFOV (1.75 TFOV for the UFF).

The one thing I would be giving up with the 20/100 is eye relief (15mm). I only have very small corrections (-0.5 Sph and -0.5 Cyl) so I think I can do visual observing without glasses. Maybe I am wrong here and I am being too optimistic. 

I totally understand that usefulness now. I just never considered  ~20mm / 100 degree eyepieces because of the massive price premium of those specs. I was aiming more towards 24mm/82 deg or 30mm/70 because they have an equivalent TFOV in my scope, better eye relief, and better prices, at the expense of more exit pupil (which wouldn't be an issue in truly dark skies, but it doesn't get better than bortle 4 for me).   

Its a trade off between contrast with the 20mm and exit pupil with the 30 mm for filter usage, see:

 

I didn't see the point of both until it was posted out by @jetstream too.

[faulksy]

On 11/04/2021 at 16:15, wookie1965 said:

Mike did you make your own or buy the hood I could do with one from home. 

just ordered 2 from https://r-sky.org/en/payment-and-shipping

also ordered 2 monk hoods of ebay last night, dont know if they will be any good though 🤣

[faulksy]

9 hours ago, amaury said:

I'll do my homework with this. 

It helps yes. The thing is that with my scope (8" 1200mm f/6 Dob), the APM 20 gives me 1.67deg TFOV, that makes the APM 30 UFF redundant as it almost overlaps in terms of TFOV (1.75 TFOV for the UFF).

The one thing I would be giving up with the 20/100 is eye relief (15mm). I only have very small corrections (-0.5 Sph and -0.5 Cyl) so I think I can do visual observing without glasses. Maybe I am wrong here and I am being too optimistic. 

I totally understand that usefulness now. I just never considered  ~20mm / 100 degree eyepieces because of the massive price premium of those specs. I was aiming more towards 24mm/82 deg or 30mm/70 because they have an equivalent TFOV in my scope, better eye relief, and better prices, at the expense of more exit pupil (which wouldn't be an issue in truly dark skies, but it doesn't get better than bortle 4 for me).   

just consider contrast, the 20mm 100* will be a lot better over the 30mm 70*

[amaury]

8 hours ago, Deadlake said:

Its a trade off between contrast with the 20mm and exit pupil with the 30 mm for filter usage

Yes and no.

For extended objects, contrast is independent of exit pupil. 

Magnification can make you see more detail due to the increased resolution so it seems that contrast improved, it is actually still the same contrast (for extended objects).

That's is the real benefit of observing the same TFOV but with a higher magnification. 

There is this good article that explains it better than I do.

https://medium.com/@phpdevster/how-telescope-aperture-affects-your-view-24507147d7fc

What you are really giving up with the 20mm/100deg EP is eye relief. I believe there isn't a single 20mm/100deg eyepiece that is eyeglasses-friendly. On the other hand, the APM UFF 30mm has superb eye relief.

[Stu]

1 hour ago, amaury said:

What you are really giving up with the 20mm/100deg EP is eye relief. I believe there isn't a single 20mm/100deg eyepiece that is eyeglasses-friendly. On the other hand, the APM UFF 30mm has superb eye relief.

I think you are perhaps missing the significance of the difference in exit pupil between these two. It’s not all about TFOV or mag.

As @Deadlake says, the 30mm would give benefits from larger exit pupil which will likely give better views with UHC or OIII filters. The 20mm will give you more image scale which helps detect the smaller objects and also does darken the sky background when you have more light pollution to contend with. I accept that it also dims extended objects but object visibility is a complex subject and your statement that contrast is independent of exit pupil is not the whole story, see this visibility calculator from Mel Bartels who has forgotten more about this stuff than I will ever know 😉.

https://www.bbastrodesigns.com/VisualDetectionCalculator.htm

[johninderby]

Interesting thread. I’ve found that it isn’t as simple as just exit pupil size but there are other factors as mentioned above that affect the views. I just consider it as one of the factors not the only factor. 

[amaury]

Thanks @Stu, as @johninderby said, using exit pupil alone or any variable alone is an over simplification of a very complex subject such as the visibility of a given astronomical object. 

It's a never ending balancing act of many variables. I guess that explains why people have many eyepieces 😅  

The general consensus seems to be that the 20mm is more useful in most instances. Something tells me I'll buy both 😆

You guys have any insight about the ES82 24mm? it seems to be a happy middle, a quick search tells me it's apparently the weakest of the ES82 family. 

[jetstream]

1 hour ago, amaury said:

For extended objects, contrast is independent of exit pupil. 

Contrast is fixed and is the difference between sky brightness and the surface brightness of the object.

[Deadlake]

38 minutes ago, Stu said:

I think you are perhaps missing the significance of the difference in exit pupil between these two. It’s not all about TFOV or mag.

As @Deadlake says, the 30mm would give benefits from larger exit pupil which will likely give better views with UHC or OIII filters. The 20mm will give you more image scale which helps detect the smaller objects and also does darken the sky background when you have more light pollution to contend with. I accept that it also dims extended objects but object visibility is a complex subject and your statement that contrast is independent of exit pupil is not the whole story, see this visibility calculator from Mel Bartels who has forgotten more about this stuff than I will ever know 😉.

https://www.bbastrodesigns.com/VisualDetectionCalculator.htm

Thanks @Stu, i’ll remember that contrast is on the hot list of subjects like the glass question etc..😃😀😃